1. Field of the Invention
The present invention relates to a wideband capable directional coupler.
2. Description of the Related Art
Directional couplers are used for detecting the levels of transmission/reception signals in transmission/reception circuits of wireless communication apparatuses such as cellular phones and wireless LAN communication apparatuses.
A directional coupler configured as follows is known as a conventional directional coupler. The directional coupler has an input port, an output port, a coupling port, a terminal port, a main line, and a subline. One end of the main line is connected to the input port, and the other end of the main line is connected to the output port. One end of the subline is connected to the coupling port, and the other end of the subline is connected to the terminal port. The main line and the subline are configured to be electromagnetically coupled to each other. The terminal port is grounded via a terminator having a resistance of 50Ω, for example. The input port receives a high frequency signal, and the output port outputs the same. The coupling port outputs a coupling signal having a power that depends on the power of the high frequency signal received at the input port.
Major parameters indicating the characteristics of directional couplers include coupling, isolation, and return loss at the coupling port. Definitions of these parameters will now be described. First, assume that the input port receives a high frequency signal of power P1. In this case, let P3 denote the power of the signal output from the coupling port. Further, assuming that the output port receives a high frequency signal of power P02, let P03 denote the power of the signal output from the coupling port. Assuming that the coupling port receives a high frequency signal of power P5, let P6 denote the power of the signal reflected at the coupling port. Further, let C denote coupling, I denote isolation, and RL denote return loss at the coupling port. These parameters are defined by the following equations.C=10 log(P3/P1)[dB]I=10 log(P03/P02)[dB]RL=10 log(P6/P5)[dB]
The coupling of the conventional directional coupler increases with increasing frequency of the high frequency signal received at the input port. The conventional directional coupler thus suffers from the problem that the frequency response of the coupling is not flat. Where coupling is denoted as-c (dB), an increase in coupling means a decrease in the value of c.
U.S. Patent Application Publication Nos. 2012/0319797 A1 and 2015/0236666 A1, and JP 2013-214840A each disclose a directional coupler aiming to resolve the aforementioned problem. The directional coupler disclosed in U.S. Patent Application Publication No. 2012/0319797 A1 includes a main line, first and second sublines electromagnetically coupled to the main line, and a phase conversion unit provided between the first and second sublines. The phase conversion unit causes a phase shift to be generated in a signal passing therethrough in such a manner that the absolute value of the phase shift monotonically increases within the range from 0 degree to 180 degrees as the frequency increases in a predetermined frequency band. The phase conversion unit is specifically a low-pass filter.
The directional coupler disclosed in JP 2013-214840A includes a main line and a subline. The subline includes two coupling parts each configured to be electromagnetically coupled to the main line, and a non-coupling part provided between the two coupling parts. The non-coupling part can be said to be a delay line formed of a long line.
The directional coupler disclosed in U.S. Patent Application Publication No. 2015/0236666 A1 includes a main line, first and second subline sections each configured to be electromagnetically coupled to the main line, and a matching circuit provided between the first and second subline sections.
All-pass circuits composed of LC circuits are disclosed in “Electronic Filter Design Handbook” by Arthur B. Williams, McGraw-Hill, New York, 1981, pages 7-1 to 7-9.
Mobile communication systems conforming to the Long Term Evolution (LTE) standard have become practically used in recent years, and further, practical use of mobile communication systems conforming to the LTE-Advanced standard, which is an evolution of the LTE standard, is under study. Carrier Aggregation (CA) is one of the key technologies of the LTE-Advanced standard. CA uses multiple carriers called component carriers simultaneously to enable wideband transmission.
A mobile communication apparatus operable under CA uses multiple frequency bands simultaneously. Accordingly, such a mobile communication apparatus requires a wideband capable directional coupler, that is, a directional coupler usable for multiple signals in multiple frequency bands.
A sought-after characteristic of wideband capable directional couplers is that a change in coupling in response to a change in frequency be reduced over a wide frequency band. In this respect, a satisfactory characteristic has not necessarily been obtained with the directional couplers disclosed in U.S. Patent Application Publication Nos. 2012/0319797 A1 and 2015/0236666 A1, and JP 2013-214840A.